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Wednesday, 15 January 2014

Tip-toe through the (African) tulips

Dr Richard Markham, ACIAR’s Pacific Crops research program manager, recently attended a planning workshop in Fiji on managing weeds in the Pacific islands. Participants debated the conundrum of how to deal with a species, such as the African tulip tree, that is valued by some and loathed by others...

Fiji's northern coast is dominated by introduced plant species -
some are vital economic species and some are weeds

Visitors to Fiji and other Pacific islands often admire the 'beautiful, open landscapes’ of the northern and western sides of the larger islands. Protected here from the rain-bearing trade winds, rain forest gives way to grassy hills, and orderly farms of cereals, legumes and sugarcane. Ecologically minded folk, however, realise that this landscape is created by human intervention and dominated by exotic species (introduced from other parts of the world), including crops and the weeds that arrived with them. The original forest has gone and nobody knows how many species of indigenous plants and insects disappeared with it.

Researchers met recently in Nadi, Fiji, and wrestled with the challenge of managing weeds and the conundrum that some plants, like the African tulip tree (Spathodea campanulata), may be a weed to one person but a useful species to another. Previously researchers have tended to accept the perspective of agriculturalists and foresters that plants invading their plantations are 'weeds' and need controlling, as cost-effectively as possible.

Our preferred approach has been to use 'classical biological control'—intentionally introducing carefully selected natural enemies from the area of origin of the nuisance species itself. Biocontrol specialists select only species that are so closely adapted to the target weed that they would (literally) rather die than feed on anything else. In that way, unforeseen and undesirable side effects are avoided.

Galls formed by larvae of an intentionally introduced fly, Cecidochares connexa, stunt the growth of Siam
weed in Papua New Guinea

Some notable successes have been achieved. For instance, in a recent ACIAR-funded project in Papua New Guinea, Siam weed (Chromolaena odorata, a species originally from Latin America and the Caribbean and now a troublesome weed across tropical Asia and Africa), has been largely brought under control by the introduction of a stem-galling fly, Cecidochares connexa. This has allowed native forest to regenerate and smallholders to replant food gardens that had been over-run.

However, in the early days of biological control, mistakes were made. When Fiji's leading conservationist, Dr Dick Watling, talked to the workshop participants, he challenged them with Fiji's negative experiences. For instance, the Indian mongoose was introduced in 1883 to control rats and has subsequently been blamed for the local extinction or reduced populations of various ground-nesting birds.

Thickets of African tulip trees help stabilise denuded hillsides

But biological control has advanced enormously in recent decades.
Scientists cannot afford mistakes and they now have the tools, including
rigorous specificity testing, to avoid them.

Retrospectively, we can’t tell which impacts on native species were due to intentional introductions and which to broader human impacts, including forest clearing and agriculture. However, on a precautionary principle, regulations relating to all exotic species imports, including biological control agents, are now much stricter. Moreover, a wider range of stakeholders now expect to be consulted in decisions about whether species should be introduced or not.

The workshop debated these issues in relation to what to do about the African tulip tree. From a Fijian agriculturist's perspective, the case is relatively simple: foreigners brought in this species for its supposedly attractive flowers, but its abundance of wind-borne seeds allow it to invade almost every fallow plot in the country. Its wood is wet and spongy, so useless for firewood or construction. The dense thickets it establishes after a few years are extremely hard to clear. Farmers want this invader removed.

Dr Stefan Neser shares his experiences of seeking biological
control agents of African tulip tree in West Africa

On the other hand, as Dr Watling pointed out, there is no evidence that the African tulip tree invades native forest or displaces native plant species. It only readily establishes itself in habitats where humans have already destroyed the natural vegetation, through repeated burning or cultivation. And in these situations, the tree helps to reforest denuded land, stabilising hillsides, reducing soil erosion and combating excessive run-off, of the kind that has led to devastating floods in recent years in the Nadi basin.

A pathway to reconciling the interests of agriculturalists and conservationists was offered by Dr Stefan Neser from South Africa. In that country, classical biological control has been used with almost surgical precision, to reduce the impact of foreign species on valued natural habitats. It has also been used to reduce the 'aggressiveness' of potentially invasive species, including useful timber species. Dr Neser believes that, among the plethora of insects feeding on the tulip tree in Africa, it should be possible to find species that attack only the flowers and seeds of the tulip. Such an insect could reduce the quantities of seeds drifting across the Pacific islands.

The workshop decided to pursue studies in West Africa, where the African tulip tree originates, to look for specific natural enemies with the desired characteristics. In the meantime, some research should also be done in Fiji, to better understand the role of this tree in Pacific island ecology. Discussions will continue to establish a 'best practice' of consulting a wider range of stakeholders. In this way, we expect that once the research in West Africa yields results, we shall have the scientific basis we need to agree on future introductions.

More information:
ACIAR success stories on biological control:
Project CP/1996/091 Biological control of Chromolaena odorata in Indonesia and Papua New Guinea News article Weed succumbs to biocontrol in PNG, shows long term study Conference abstract Biocontrol of Chromolaena odorata in Papua New Guinea

2 comments:

workshop chose to seek after studies in West Africa, where the African tulip tree begins, to search for particular common foes with the coveted aspects. Meanwhile, some exploration might as well likewise be carried out in Fiji, to better comprehend the part of this tree in Pacific island biology.